The present invention relates to a pseudo-color image outputting system for reading a full-color image such as a color photograph and for outputting a pseudo color image represented by dots with a fixed dot diameter.
Dot printers printing an image by dots with a fixed dot diameter can print a color image by using color materials of Cyan (C), Magenta (M) and Yellow (Y) but cannot print an image with a gray scale (gradation). For this reason, various pseudo-color gray-scale representation methods have been developed for obtaining an output approximate to a full color image using this type of dot printers.
Fundamentally, the pseudo-color gray-scale representation methods represent various colors by changing total areas of dots of each color per unit area. More specifically, a pseudo-color gray-scale representation is implemented by digitizing three primary colors of Red (R), Green (G) and Blue (B) and color-converting into three primary colors in a CMY system represented by an output unit.
A method in which a meshed-pixel distribution method is applied to a color image has been known as the pseudo-color gray scale representation method. ("A pseudo full-color representation method taking account of a color reproduction": Yamada et al, The paper of Information Processing Society of Japan, June 1987 Vol. 28, No. 6, pp 617). According to this method, a color which is actually represented by a dot pattern of a mesh constituted by 2.times.2 dots output from a dot printer is obtained by referring to a preliminarily prepared lookup table (LUT). Then, an error between the color obtained from the lookup table and the color to be represented is compensated with the neighboring meshes.
This method has an advantage that an error between an actually output color and a color to be actually represented can be compensated without performing sophisticated color-mixing calculations. This method, however, requires controls of distribution and concentration of dots in order to prevent the resolution from being lowered, since the process is performed in units of meshes each constituted by 2.times.2 dots. Further, since an accumulation of errors will deteriorate image quality, an additional control is required. These controls will complicates the entirety of the pseudo-color gray-scale representation process. Further, areas accumulated errors of which cannot be fully controlled are caused.
In order to solve the above problem, a method has been developed in which a three-dimensional (3-D) color compensation table is prepared by using a color compensation technology of the meshed-pixel distribution method and the digitizing process is performed in units of dots by performing the color compensation using the 3-D color compensation table. ("A method for preparing a 3-D color compensation table taking account of an image I/O device in a pseudo-color gray-scale representation": Moritani et al., Institute of Electric Field Hokkaido-branch Federation Conference Lecture Papers for the fourth year of Heisei, October 1992, pp 443).
However, an application of the above-described pseudo-color gray-scale representation method to actual systems will pose the following problems.
In the actual systems, the input characteristics of the image input units (image scanners) and ink characteristics of image output units (printers) vary depending on their types. For this reason, a user must prepare the 3-D color compensation table as an initial setting process, depending on the types of the image input unit and the image output unit to be used. This initial setting process is troublesome and a user's burden is heavy. Further, in a system in which a plurality of scanners are connected to a single printer or a plurality of printers are connected to a single scanner, the above-described 3-D color compensation table must be prepared each time the scanner or the printer to be used is switched, resulting in undesirable system operation.